The present invention relates to a circulating fluidized bed combustion system and particularly to a system for separating and returning the solid particles to the combustor.
One of the main features of fluidized bed combustion is the ability to burn high-sulfur fuels in an environmentally acceptable manner without the use of flue-gas scrubbers. In fluidized bed combustion, much of the sulfur contained in the fuel is removed during the combustion by a sorbent material in the fluid bed, usually limestone.
One type of fluidized bed combustion is the circulating fluidized bed system. In this system, the gas velocities in the combustor are three to four times as high as in a bubbling fluidized bed system. The small solid particles of limestone are carried up through and out of the combustor and a uniform lower-density gas/solids mixture exists throughout the entire system. Since the solids move through the system at a lower velocity than the gases, significant solids residence times are obtained. The long residence time coupled with the small particles size produce high combustion efficiency and high sulfur oxide removal with lower sorbent limestone feed.
In a circulating fluidized bed combustion system, the solids which are carried from the combustor are separated from the gas and returned to the combustor. Normally, this has been accomplished by cyclone separators which are external from the combustor and connected by a horizontal gas pass. These cyclone separators have been water cooled, steam cooled or refractory lined. These external cyclone separators require considerable building space and create differential thermal expansion problems among the combustor, the horizontal gas pass and the cyclones.